I have wondered for a while about a similar potential situation. What would the legal fallout be if someone dressed in a bigfoot costume in a forest were to be shot? Would the shooter be charged with murder or manslaughter, on the grounds that he should have known that bigfoot does not exist, which is the overwhelming consensus among zoologists? Or would he be charged with some lesser offense, or not at all, because he legitimately believed himself to be killing a non-human creature, even if such a belief is not scientifically supported? I don't know enough about criminal law to provide an answer, but someday someone may have to provide the answer in real life.